1. Field of the Invention
This invention relates to the synthesis of carbon nanostructures such as graphene, fullerenes and nano-tubes, and more particular to the synthesis of such nanostructures from carbon-excess explosives in supercritical fluid.
2. Description of the Related Art
Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb, hexagonal crystal lattice. Graphene is a basic building block for graphitic materials of all other dimensionalities. Graphene can be wrapped up into 0 D fullerenes, rolled into 1 D nanotubes or stacked into 3 D graphite. Techniques for epitaxy deposition of graphene include but are not limited to Molecular Beam Epitaxy (MBE), Chemical Vapor Deposition (CVD) and plasma assisted CVD.
Graphene has rather remarkable properties. Graphene is stable, chemically inert, and crystalline under ambient conditions. It is a semimetal in that its conduction and valence bands just meet at discrete points in the Brillouin zone. An electron in graphene has an effective mass of zero and behaves more like a photon than a conventional massive particle. Finally graphene can carry huge current densities—about 108 A/cm2, roughly two orders of magnitude greater than copper. Graphene is a candidate for replacing silicon as a basis for faster, more powerful electronics. Graphene nanoribbons (GNRs) are essentially single layers of graphene that are cut in a particular pattern to give it certain electrical properties.
A fullerene is a spherical shaped carbon molecule. A common method used to produce fullerenes is to send a large current between two nearby graphite electrodes in an inert atmosphere. The resulting carbon plasma arc between the electrodes cools into sooty residue from which many fullerenes can be isolated.
A carbon nanotubes (CNT) is a hollow cylindrical shaped carbon molecule. The properties of single-walled nanotubes (SWNTs) are determined by the graphene structure in which the carbon atoms are arranged to form the cylinder. Multi-walled nanotubes (MWNTs) are made of concentric cylinders around a common central hollow. CNTs have stimulated a great deal of interest in the microelectronic and other industries because of their unique properties including tensile strengths above 35 GPa, elastic modulus reaching 1 TPa, higher thermal conductivity than diamond, ability to carry 1000× the current of copper, densities below 1.3 g/cm3 and high chemical, thermal and radiation stability. CNTs have great promise for devices such as field effect transistors, field emission displays, single electron transistors in the microelectronic industry, and uses in other industries. CNTs are commonly grown using several techniques such as arc discharge, laser ablation and chemical vapour deposition (CVD). Commercialization of CNTs will depend in large part on the ability to grow and network CNTs on a large cost-effective scale without compromising these properties.
Another proposed approach is to change waste carbon dioxide into CNTs. In this approach supercritical carbon dioxide (scCO2) is used as the carbon source and alkali metals (Li or Na) as the reductants to synthesize CNTs at reaction temperatures of 600-750 degrees C. The Lithium reacts with the supercritical CO2 to produce Lithium-Carbonate and activated carbon. The activated carbon reassembles or self-organizes into CNTs. In this processes the CO2 is at least partially consumed by the synthesis of the CNTs. This approach is offered as a technique for synthesizing CNTs that can be scaled up for industrial applications. However, the use of alkali metals and the high reaction temperatures increases the cost of the process. Furthermore, the presence of a metal reactant will leave metal contaminants in the extracted CNTs, which is undesirable for certain applications such as nano-electronic devices. See Zhengsong Lou et al. “Synthesis of carbon nanotubes by reduction of carbon dioxide with metal lithium” Letters to the Editor, Carbon 41 (2003) 3063-3074 and Zhengsong Lou et al. “Formation of variously shaped carbon nanotubes in carbon dioxide-alkali metal (Li, Na) system” Letters to the Editor, Carbon 43 (2005) 1084-1114, which are hereby incorporated by reference.